The Anti-Afterfire Solenoid

What this little device does is mistaken by many people, like the governor, so I will explain its purpose and how it works.

The correct term is the “Anti-Afterfire Solenoid”. It does exactly that, prevents “afterfiring” when the engine is cut off. This is not to be confused with backfiring, which is a totally different thing. Afterfiring is that loud bang you hear after cutting off a tractor engine. This condition is more popular in Briggs and Stratton engines than any other engine. The loud bang is totally harmless to the engine, but can be scarry or annoying to the operator.

What causes the “afterfiring” is raw fuel touching the hot internal parts of the muffler. How this works is when you cut off the engine after mowing the grass, the engine “coasts” to a stop (keeps spinning until it slows down and stops). When the engine is coasting, the fuel is still being drawn into the cylinder on the intake stroke, compressed on the compression stroke, and exhausted on the exhaust stroke. The key here is the fact that the fuel is not combusted under the compression stroke (as it would be when the engine is on). The fuel takes a ride through the engine and ends up in the muffler. When this raw, unburned fuel hits the muffler, it explodes (like it would during the compression stroke). The fuel continually “rides” through the engine until it stops coasting. By the time the engine stops, there may be four or five compression stroke’s worth of fuel in the muffler, ready to blow up.

Now the job of the solenoid is to prevent this from happening. It is a very simple, yet expensive part. It works with electromagnatism and a spring. The spring constantly keeps the metal pintle (finger) in the extended position until the ignition switch is turned on. When the swtich is on, 12 volts is supplied to the solenoid, which makes the electromagnet pull down on the pintle. When the switch is off, the spring automatically returns the pintle to the extended position. If you have seen an IAC valve on a car, you will have an idea of what I am talking about.

When you have the solenoid installed on the bottom of the carburetor and turn off the engine, the spring pushes up on the pintle. This pintle actually blocks the main nozzle, preventing fuel from being sucked up through it when the engine is coasting to a stop, thus preventing the loud bang noise when the engine stops.

If you don’t have this solenoid, you can prevent the afterfire noise by slowing the engine down to idle and let it cool off for about 30 seconds. This will give the muffler a chance too cool.

It is possible for the engine to still afterfire if you do have the solenoid. To correct this problem, cut the engine off in fast. DO NOT IDLE DOWN.

Home | Questions?
updated 11-14-04

7 responses to “The Anti-Afterfire Solenoid”

  1. Mower Owner says:


    Thanks for the info. My question is where does the gray wire get wired to? I see that the black wire is grounded on the engine block, but I’m not sure of the other wire.

    Could you help please.

    Mower Owner

  2. Bill says:

    Got this from another site talking about these.

    “This solenoid must be connected to 12 volts while the engine is running and no voltage when the key switch is turned off. It can come 2 ways: The old style used the solenoid case as the ground, which automatically grounded the solenoid when it was screwed into the carburetor and the black wire went to the key switch. Presently, there are 2 wires, the gray wire goes to the key switch and the black wire goes to ground.”

  3. Jim Lindley says:

    I purchased a used mower and I saw this device on my carb. I took my carb apart to investigate an unrelated “crankcase flooding with gas” issue. What is interesting is that on my used mower, this anti-afterfire solenoid is not connected to any wires, as if the original wires have been removed. I noticed on the engine block there is a wire harness connection “blade” or male end where a wire could attach. What is particularly confusing is that the “solenoid plunger” (if viewed on the inside of the bottom of the bowl” is wedged shape and does not appear it would fully engage into the hole in the float device.
    I have not had any “bangs” when turning off my mower, and I did notice that the wedge plunger was stuck, and I am soaking to free it, but after reading this I realize it is really non-functional since no power is going to it. I am guessing that if you are not having any after fire issues, this device is really not needed.
    What do you think?

    • William McInnes says:

      This is interesting. I’ve experienced 2 problems. The first is that the cylander floods with gasoline whan the engine is not running. I solved this problem bu installing a shutoff valve to the fuel line. The second is afterfiring. It sounds as though the real problem is likely the solenoid.

  4. Robert Smith says:

    I have a cheap Chinese 13hp generator with one, strangely when troubleshooting the afterfire sol it is in retracted with no power but extends with 12v applied. It had been really gummed up and cleaned it. Wiring dia shows one wire going to key, other to “commutate” unit, with switch on and battery installed do not see voltage on either one to ground. Switching to “newer style carb with manual choke and no valve possibly.

  5. tom ho says:

    Indeed that is exactly what it is but i have one more as i have the Toro LX425 with flooded carb. to start with due to bad float needle then later spread to bad solenoid fuel shut off. I went away to install the manual fuel shut off valve but that thing leaks after a short while now I am thinking of buying the whole carb. (cheaper) so that to have a new float needle valve, solenoid and the rest.

    Here is the one more thing. It will prevent gas odor leakage with the solenoid in place. I know real well because the whole garage smells when i park it there.

    Solution is to run it until it uses up all fuel in bowl then park it.

    So you see to have the fuel solenoid working has two advantages:

    1. prevent pop noise when shut off
    2. gas odor from escaping to the environment which is bad emission

    The manual shut off valve is cheap so i just put in another new one as it cut off fuel to carb. completely to prevent flooding with bad needle float. It will happen when you least expect.
    I might put in another new carb. later so i have the solenoid again working to avoid the nuisances already described.

  6. Rich Potts says:

    To which pin on the starter switch does the gray wire connect to?

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